Wearable computing platforms such as health-tracking and head-mounted systems present new challenges to energy-efficient design. Unlike desktop and mobile systems, such platforms function primarily with their displays (if any) turned off. They spend a majority of their time reading data from sensors such as pressure sensors in elevation monitoring, accelerometers and gyroscopes in step counting applications, color/light sensors in pulse oximeter health applications, and cameras in head-mounted augmented-reality systems.
Sensor power dissipation is important. The processors in wearable, embedded, and mobile platforms are usually the main focus of power-reduction efforts. These processors are, however, often connected to many sensor integrated circuits (ICs). Because the power-efficiency for inter-IC communication may be limited by printed circuit board properties, power consumption of sensor circuits has not scaled with semiconductor process technology and packaging advances. As a result, the power dissipated in some state-of-the-art sensors may be nearly as high as the power dissipation of low-power processors.
Input/output (I/O) energy costs generally range from 10 fJ/bit/mm to 180 fJ/bit/mm in on-chip links, to between 2 pJ/bit and 40 pJ/bit for typical printed circuit board (PCB) traces. At data rates of 1 Mb/s typical of modern embedded serial links, these energy costs per bit lead to I/O power dissipation between 2 μW and 40 μW. This may represent a significant portion of power dissipation of a processor and such power dissipation is incurred for each sensor in a system. I/O energy thus presents a significant portion of power usage in many low-power embedded systems.
To enable smaller device packages, smaller PCB designs, and lower costs, the inter-IC communication links in many embedded computing platforms are bit-serial and not parallel buses. Prior efforts to reduce communication power by encoding data using techniques such as Gray coding have, however, targeted parallel buses and are not applicable to reduce data transfer power in bit-serial communication interfaces. Therefore, there is a need in the industry to address one or more of the above mentioned shortcomings.